As generally known, in an MR imaging (MRI) system or MR scanner, an examination object, usually a patient, is exposed to a uniform main magnetic field (B0 field) so that the magnetic moments of the nuclei within the examination object form a certain net magnetization of all nuclei parallel to the B0 field, which can be tilted leading to a rotation around the axis of the applied B0 field (Larmor precession). The rate of precession is called Larmor frequency which is dependent on the specific physical characteristics of the involved nuclei, namely their gyromagnetic ratio, and the strength of the applied B0 field. The gyromagnetic ratio is the ratio between the magnetic moment and the spin of a nucleus.
By transmitting an RF excitation pulse (B1 field) which is orthogonal to the B0 field, generated by means of an RF transmit antenna or coil, and matching the Larmor frequency of the nuclei of interest, the spins of the nuclei are excited and brought into phase, and a deflection of their net magnetization from the direction of the B0 field is obtained, so that a transversal component in relation to the longitudinal component of the net magnetization is generated.
After termination of the RF excitation pulse, the relaxation processes of the longitudinal and transversal components of the net magnetization begin, until the net magnetization has returned to its equilibrium state, wherein T1 and T2 are the times required for the longitudinal and transversal magnetization, respectively, to return to 63% of their equilibrium values. MR signals which are generated by the precessing magnetization, are detected by means of an RF receive antenna or coil. The received MR signals which are time-based amplitude signals, are then Fourier transformed to frequency-based MR spectrum signals and processed for generating an MR image of the nuclei of interest within an examination object.
In order to obtain a spatial selection of a slice or volume within the examination object and a spatial encoding of the received MR signals emanating from a slice or volume of interest, gradient magnetic fields are superimposed on the B0 field, having the same direction as the B0 field, but having gradients in the orthogonal x-, y- and z-directions. Due to the fact that the Larmor frequency is dependent on the strength of the magnetic field which is imposed on the nuclei, the Larmor frequency of the nuclei accordingly decreases along and with the decreasing gradient (and vice versa) of the total, superimposed B0 field, so that by appropriately tuning the frequency of the transmitted RF excitation pulse (and by accordingly tuning the resonance frequency of the RF/MR receive antenna), and by accordingly controlling the gradient magnetic fields, a selection of nuclei within a slice at a certain location along each gradient in the x-, y- and z-direction, and by this, in total, within a certain voxel of the object can be obtained.
The above RF (transmit and/or receive) antennas can be provided both in the form of so-called body coils (also called whole body coils) which are fixedly mounted within an examination space of an MRI system for imaging a whole examination object, and as so-called surface or local coils which are arranged directly on or around a local zone or area to be examined and which are constructed e.g. in the form of flexible pads or sleeves or cages like head coils.
Further, such RF transmit and/or receive antennas can be realized on the one hand in the form of an RF antenna array or array coil, which comprises a number of individual coils or coil elements which are individually selected for being driven by an own RF current source in order to generate (and/or receive) their own local magnetic field such that a desired overall magnetic field distribution is generated within (or received from) the examination space by all coil elements together. However, this requires that the individual coils or coil elements are electromagnetically decoupled from each other, or the mutual couplings (mainly due to magnetic flux) between the elements are compensated.
On the other hand, such RF transmit and/or receive antennas can be realized in the form of an RF resonator, especially an RF volume resonator, also called RF volume coil, which comprises a conductor structure with a number of conductor elements which electromagnetically couple to each other such that by driving the RF resonator at one or two ports by an RF current source, a number of linearly independent resonant current distributions (“resonant modes”) can be excited in the RF resonator for generating magnetic fields at certain resonance frequencies in a volume of interest (usually an examination space).
Such RF resonators are known especially in the form of birdcage type RF coils and TEM type coils. Both can comprise a conductor structure in the form of a number of longitudinal conductor elements which are arranged in parallel to each other in a cylindrical pattern having a circular or an oval or elliptical or other cross sectional shape such that a substantially cylindrical volume for receiving an examination object or a part thereof is enclosed by the conductor structure. The longitudinal conductor elements are usually rungs or strip lines (especially each in the form of a longitudinal conductive coating on a printed circuit board or another carrier) which in case of a birdcage type RF coil are conventionally galvanically connected to each other at both axial ends of the coil e.g. by means of electrically conducting end caps or circular or oval or elliptical or other electrical loop conductors. Preferably, a cylindrical RF shield is provided which coaxially surrounds the conductor structure and is disconnected from the same, wherein the RF shield is provided for preventing the surroundings from being exposed to the RF fields generated within the birdcage type RF coil.
In case of a TEM type coil, the longitudinal conductor elements are usually not galvanically connected to each other. Instead, the conductor elements are coupled at their axial ends and/or at one or more positions along their length by means of one or more capacitors (or galvanic conductors) to an especially cylindrical RF shield which preferably has the same cross sectional shape as the cylindrical pattern of the conductor structure and which coaxially surrounds the conductor structure in a known manner. Consequently, and in contrary to a birdcage type RF coil, this (outer) cylindrical RF shield functions as an active element which provides a return path for the currents in the (inner) longitudinal conductor elements. By this, the TEM resonator substantially behaves like a longitudinal multi-conductor transmission line which is capable of supporting standing waves at certain resonance frequencies. A separation of the resonance modes can be obtained by adjusting mutual couplings between the (inner) conductor elements, wherein by adjusting the capacitances of the capacitors which couple the conductor elements to the RF shield, the RF field distribution within the TEM resonator can be adjusted for obtaining the best field homogeneity. Again, the cross sectional shape of a TEM type coil (TEM resonator) can be circular or oval or elliptical or can have other shapes.
WO 2008/037636 (US 2010/0036237) discloses a detector unit for arrangement within a magnet field generating unit of an MRI device, comprising an RF/MR transceiver system in the form of a birdcage resonator which is divided into two part systems which are spaced apart from one another in the direction of the tunnel axis so as to form an essentially annular cavity between them. The detector unit further comprises an RF screen for shielding the RF/MR transceiver system to the outside. The cavity between the two part systems is provided for receiving a supplementary element for influencing the main magnetic field or the gradient magnet fields, or a PET detector, wherein such an element or detector is arranged in a radial direction outside of the RF screen.